The behavior of 4-aminophthalimide (4-AP), a common molecular probe utilized in solvation dynamics experiments, was revisited in polar aprotic and protic solvents using absorption, steady-state, and time-resolved fluorescence (TRES) techniques. Also, the deuterium isotope effect was investigated using D(2)O as solvent. The absorption spectra of 4-AP consist of two absorption bands with maxima around 300 nm (B2 band) and 370 nm (B1 band) depending on the environment, while the emission feature consists of a single band. In all the solvents investigated (excluding water), the 4-AP photophysics is similar and the emission spectra are independent of the excitation wavelength used. In water the behavior is unique and the emission spectra maximum is different depending on the excitation wavelength used. The emission maximum is 561.7 nm using the excitation wavelength that correspond to the B2 absorption band maximum (λ(excB2) = 303.4 nm) but is 545.7 nm when the excitation wavelength that correspond to the B1 absorption maximum (λ(excB1) = 370.0 nm) is used. Moreover, while the fluorescence decays of 4-AP in water exhibit no emission wavelength dependence at λ(excB2), the situation is quite different when λ(excB1) is used. In this case, we found a time-dependent emission spectrum that shifts to the blue with time. Our results show that the solvent-mediated proton transfer process displays a fundamental role in the 4-AP emission profile and for the first time a mechanism was proposed that fully explains the 4-AP behavior in every solvent including water. The deuterium isotope effect confirms the assumption because the proton-transfer process is dramatically retarded in this solvent. Consequently, we were able to elucidate not only why in water the emission spectra depend on the excitation wavelength but also why the time-dependent emission spectra shift to the blue with time. Thus, our work reveals the importance that the medium has on the behavior of a widespread dye used as chromophore. This is significant since the use of chromophores without understanding its chemistry can induce artifacts into the interpretation of solvation dynamics in heterogeneous environments, in particular, those provided by aqueous biological systems.

No abstract

The reactions of n‐butylamine and piperidine with 1‐chloro‐2, 4‐dinitrobenzene and 4‐chloro‐3‐nitrotrifluoromethylbenzene were studied in n‐hexane, benzene and mesitylene. The reactions seem to follow a bimolecular SNAr mechanism for both substrates and amines. However, whereas in n‐hexane they are base catalysed, in the aromatic solvents only mild acceleration is observed. The different behaviour of these reactions in the aromatic solvents in comparison with hexane is explained by the specific solvent effect exerted by the aromatic compounds, which solvates the aromatic reactant preferentially in view of its electron‐donor properties. These conclusions were confirmed by kinetic studies in hexane‐aromatic solvent binary mixtures. It can be concluded that the presence of electron‐donor solvents may inhibit SNAr reactions when association constant of the solvent with the substrate is greater than the EDA constants between the reactants. These specific solvent effects could also explain why only mild acceleration is observed in aromatic solvents.

903 (1985). The interaction of several aliphatic amines as n-donors and dinitrobenzenes (DNB) as T-acceptors has been studied in n-hexane. The formation of electron donor -acceptor (EDA) complexes is proposed to explain the spectroscopic behaviour of the mixtures. The stability constants (K,) for these complexes have been calculated by an iterative procedure. For a given acceptor, the donor strength of RNH, > R,NH > R,N was found. This order is explained by considering the role that steric effect may play in the EDA complex formation. On the other hand, the fact that for a given donor K, follows the order I ,2-DNB > I ,3-DNB > I ,4-DNB, and that I ,2-DNB reacts with primary amines, led to the proposal of orientational complexes. These EDA complexes may be considered intermediates in aromatic nucleophilic substitution reactions. On a CtudiC I'interaction de pluseurs amines aliphatiques, comme donneurs tz et dinitrobenzens (DNB), comme accepteurs T, dans n-hexane. On propose la formation de complexes electron-donneur -accepteur (EDA) pour expliquer le comportement spectroscopique des melanges. La constante de stabilite (K,) pour ces complexes a Cte calculee au moyen d'une itCrature. Pour un accepteur donne, on a observC que la force du donneur etait RNHz > RzNH > R,N. Cela s'explique en considCrant le role que jouent les effects steriques dans la formation de complexes EDA. D'ailleurs le fait que K,, pour un donneur donne, ait I'ordre I ,2-DNB > I ,3-DNB > 1,4-DNB et que I ,2-DNB reagisse avec amines primaires, conduit a proposer des complexes orientationnels. Ces complexes EDA peuvent etre consideris des intermediaires dans les reactions de substitution nucl6ophilique aromitique.

No abstract

No abstract

No abstract

No abstract